Correct interpretation of the human electrocardiogram depends in part upon a precise understanding of disorders of ventricular excitation. In previous studies we have produced lesions in canine and primate hearts which interrupted conduction through portions of the left bundle branch without or with concurrent right bundle branch block. Electrophysiologic and hemodynamic consequences were observed. However, many questions concerning these fascicular blocks (or, "hemiblocks") remain unanswered. We proposed herein to extend our previous investigations to determine (a) the effects of left anterior fascicular block or left posterior fascicular block upon pre-existent right bundle branch block, (b) the mechanisms of axis shift in pre-existent left bundle branch block, (c) the reasons for marked species differences among animals studies thus far, and (d) the role of specific pacing characteristics or of certain physical or chemical ancillary factors upon the expression or suppression of the electrocardiographic manifestations of known anatomic intraventricular conduction disturbances.